If I desired to construct a textile factory in the United States to serve U.S. markets, I would understandably be required to comply with U.S. building and land regulation, employ U.S. persons, adhere to U.S. employment and environmental law, and naturally use U.S. construction companies to build my facility.
Absent the Jones Act, if I desired to similarly provide cabotage (U.S. port to U.S. port) cargo or passenger service, I could completely skirt U.S. laws and regulations by constructing my vessel in a low-cost Asian shipyard; hiring a foreign crew at cut-rate wages, benefits, and working conditions; avoiding U.S. maritime safety and environmental regulations; and just drive my vessel into the United States to do business in domestic waters. It is as if my factory, built by foreign labor, employing foreigners, and exempt from U.S. regulation, was plopped down in the middle of Missouri across the street from a similar factory that is subject to all the U.S. regulations. My U.S.-compliant rival would rightly complain about an unfair competitive environment!
In addition to the unique mobility issue is military readiness.